Chapter 1 · Animation 6 of 6 · Section 1.6

The MOSFET at Work

Raise the gate voltage to build a channel of electrons, then raise the drain voltage to move them. Watch the channel thin out toward the drain and pinch off in saturation.

electronhole− acceptor ion (depletion region, violet)+ gate charge

Electron charge per unit area along the channel (charge × velocity = ID is the same at every point)

ID versus VDS at this VGS and VSB (dashed: start of saturation)

What to look for

  • Count the dots near the source and near the drain: the channel thins toward the drain as VDS grows, yet the current is the same everywhere, so the electrons near the drain move faster.
  • In saturation the charge near the drain is tiny and the electrons are swept across the last stretch. Raising VDS further changes the ID–VDS curve very little.
  • With VSB > 0 the violet depletion region deepens under the whole gate and the channel holds fewer electrons.

The physics behind it

  • The gate charge is balanced by the depletion charge (ionized acceptors) plus the inversion charge (electrons). The electrons are what conduct.
  • Along the channel |Q′I| ≈ C′ox(VGS − VT − nV(x)) in strong inversion, and current continuity fixes how V(x) and the electron velocity change from source to drain.
  • The book develops these ideas qualitatively in Section 1.6 (Figs. 1.31 to 1.36); the equations come in Chapters 3 and 4.